Due to its various advantages such as that voltage is low, consumption of electricity is low and it is possible to make into small size and into thin thickness, liquid crystal display devices have been widely utilized for use as television and monitor for personal computers and portable instruments. With regard to the liquid crystal display device as such, various modes have been proposed depending upon the orientation state of liquid crystals in liquid crystal cells and, up to now, a TN mode where a twisted orientation state is about 90° from the lower-side substrate to the upper-side substrate of the liquid crystal cell has been the mainstream.
Usually, a liquid crystal display device is constituted from liquid crystal cell, optical compensation sheet and polarizer. The optical compensation sheet is used for solving the coloration of image or for expanding the visual angle and film where liquid crystal is applied to double refraction film or transparent film has been used. For example, in Japanese Patent No. 2,587,398, there is a disclosure for an art where discotheque liquid crystal is applied on a triacetyl cellulose film, oriented and fixed and the resulting optical compensation sheet is applied to a liquid crystal cell of TN mode to expand the visual angle. However, in liquid crystal device to be used for television where it is presumed to see a large screen from various angles, there is a severe demand for dependency of visual angle and the demand as such is unable to be satisfied by the aforementioned means. Therefore, liquid crystal devices being different from TN mode such as IPS (in-plane switching) mode, OCB (optically compensatory bend) mode and VA (vertically oriented) mode have been studies. Since VA mode has particularly high contrast and relatively high yield in the manufacture, it has been receiving attention as a liquid crystal display device for TV.
As compared with other polymer films, cellulose acetate film usually has a characteristic that its optical isotropy is high (retardation value is low). Therefore, it is common that cellulose acetate film is used for the use where optical isotropy is demanded such as a polarizing plate.
On the contrary, optical anisotropy (high retardation value) is demanded for an optical compensation sheet (phase contrast film) for liquid crystal display devices. Particularly for an optical compensation sheet for VA, in-plane retardation (Re) of 30 to 200 nm and thickness-direction retardation (Tth) of 70 to 400 nm are needed. Accordingly, as an optical compensation sheet, it has been usual to use synthetic polymer film having high retardation values such as polycarbonate film and polysulfone film.
As such, in the technical field of optical materials, it has been a general principle that synthetic polymer film is used in case optical anisotropy (high retardation value) is demanded for polymer film while cellulose acetate film is used in case optical isotropy (low retardation value) is demanded.
In European Patent Laid-Open No. 911656, there is a disclosure for cellulose acetate film having a high retardation value which is able to be used for the use where optical anisotropy is also demanded overturning the conventional common principles. In the document, an aromatic compound having at least two aromatic rings or, particularly, a compound having 1,3,5-triazine rings is added followed by subjecting to an stretching treatment so as to achieve a high retardation value in cellulose triacetate.
It has been known that, generally, cellulose triacetate is a polymer material which is hard to be stretched and is difficult for making its double refractive index big. However, when an additive is oriented at the same time in an stretching treatment, the double refractive index is now made possible to enlarge whereby a high retardation value is achieved. Since this film is also used as a protective film for a polarizing plate, there is an advantage that less expensive and thin liquid crystal display device is able to be provided.
In Japanese Patent Laid-Open No. 2002/71,957, there is a disclosure for optical film containing cellulose acylate having an acyl group of 2 to 4 carbons as a substituent and satisfies both of formulae 2.0≦(A+B)≦3.0 and A<2.4 in which A is degree of substitution of acetyl group and B is degree of substitution of propionyl group or butyryl group and, further, Nx which is refractive index in the direction of slow axis angle and Ny which is refractive index in the direction of progressive phase axis at wavelength of 590 nm satisfy the formula 0.0005≦(Nx−Ny)≦0.0050.
Further, in Japanese Patent Laid-Open No. 2003/270,442, there is a disclosure for polarizing plate used for a liquid crystal display device of a VA mode which is characterized in that the polarizing plate has a polarizer and an optically biaxial mixed fatty acid cellulose acylate film and that the optically biaxial mixed fatty acid cellulose acylate film is oriented between liquid crystal cell and polarizer.
Cellulose ester film is usually manufactured by a solution film-forming method. The solution film-forming method is able to manufacture film having good properties such as optical property as compared with other manufacturing method such as melting film-forming method. In the solution film-forming method, a polymer solution (hereinafter, it will be referred to as a dope) where a polymer is dissolved in a mixed solvent in which dichloromethane and methyl acetate are main solvents. The dope is made into casting beads from a casting die and casted onto a support to form a casted film. After the casted film becomes a product having a self-supporting property on the support, it is peeled from the support as a film (hereinafter, this film will be referred to as wet film), dried and wound as film (refer, for example, to Journal of Technical Disclosure No. 2001/1,745 published by the JIII (Japanese Institute for Inventions and Innovations)).
In a solution film-forming method, dry wind is applied onto the surface of the casted film in order to proceed the drying of the film. However, if drying of the casted film is carried out suddenly, there is a risk of causing the worsening of the face property of the surface. Therefore, there has been known a method where drying speed of the casted film is made not more than 300% by mass/min (=5% by mass/s) in terms of amount of the solvent contained therein on the basis of dried amount so that a slow drying is carried out (refer, for example, to Japanese Patent Laid-Open No. 11/123,732). A co-casting method where the casted film is made into a multi-layer structure has been known as well. An example is a casted film where skin layers are formed on both surfaces of a core layer which is an intermediate layer. In that case, dope viscosity of the core layer is enhanced to ensure the strength of the casted film and, at the same time, dope viscosity forming the skin layer is lowered so that flatness and smoothness of the surfaces are improved (refer, for example, to Japanese Patent Laid-Open No. 2003/276,037).
Polarizing plate is usually manufactured by adhesion of film mainly comprising cellulose triacetate as a protective film on both sides of a polarizer in which iodine or dichromatic dye is oriented to and adsorbed with polyvinyl alcohol. Cellulose triacetate film has characteristics of high toughness, flame retardant property, optical isotropy (low retardation), etc. and has been widely used as a protective film for the above-mentioned polarizing plate. A liquid crystal display device is constituted from polarizing plate and liquid crystal cell. In a TFT liquid crystal display device of a TN mode which is a mainstream of liquid crystal display devices at present, an optical compensation sheet is inserted between polarizing plate and liquid crystal cell whereby a liquid crystal display device having a high display quality is achieved as mentioned in Japanese Patent Laid-Open No. 08/050,206. However, in cellulose triacetate, much water is absorbed and permeated and, therefore, it has disadvantages that optical compensation property changes and that a polarizer is apt to be deteriorated.
Cyclic polyolefin film has been receiving public attention as a film which is able to improve moisture-absorbing and permeating properties of cellulose triacetate film and development of film for protection of polarizing plate by heat fusion film-forming and solution film-forming has been developed. Since cyclic polyolefin film has high ability for expression of optical characteristics, its development as a phase contrast film has been conducted (Japanese Patent Laid-Open No. 2003/212,927, Japanese Patent Laid-Open No. 2004/126,026 and Japanese Patent Laid-Open No. 2002/114,827).
In the method for the manufacture of cyclic polyolefin film, variations in optical characteristics in a transverse direction and in a longitudinal direction (in the directions of retardation and of optical axis) have been problems in the case of heat fusion film-forming. Another problem it that, in cyclic polyolefin film having a glass transition point (Tg) of as high as 200° C. to 400° C., temperature upon heat fusion film formation is high and, therefore, it is difficult to prepare a uniform film by means of a strict control.
On the other hand, in the solution film formation of cyclic polyolefin film, although there is an advantage that polymer having a high Tg is able to be made into film, there are problems in adjustment in expressing ability of optical characteristics and a control of variation thereof. In addition, in the use as an optical compensation film which has been used in liquid crystal display devices in recent years, there has been an increasing demand particularly for flatness of the film but, under the conventional drying conditions, it has been found to cause a problem that unevenness in stripes and in spots is generated upon drying due to its wind velocity, etc. Further, when resistance upon peeling is high, there is a problem of generation of step-like unevenness upon peeling. Particularly, unevenness in stripes and in spots which is generated in drying is a big problem causing deterioration of quality of optical films where an excellent flatness is demanded and there is a necessity for improving it.